U.S. patent number 4,811,329 [Application Number 07/058,879] was granted by the patent office on 1989-03-07 for optical head with laser having particular source of driving current.
This patent grant is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Mitsushige Kondo, Shinsuke Shikama, Eiichi Toide.
United States Patent |
4,811,329 |
Shikama , et al. |
March 7, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Optical head with laser having particular source of driving
current
Abstract
An optical type head device comprises a laser beam source for
emitting a laser beam, a light focussing means for converging the
laser beam on a data storing surface of an optical type data
storing medium, a laser beam detecting means which receives the
laser beam reflected by the optical type data storing medium and
modulated by data held by the same, and converts the data into
electric signals, a driving current supplying means for supplying a
driving current to the laser beam source. The driving current
supplying means comprises a high frequency current generator which
supplys to the laser beam source a high frequency current having a
waveform ranging from the lower part of the threshold value at
which the laser beam source starts emission of the laser beam to
the higher part of the thresheld value, and the frequency of the
high frequency current is at least twice as high as the maximum
frequency component of information which is contained in the data
storing medium.
Inventors: |
Shikama; Shinsuke (Nagaokakyo,
JP), Toide; Eiichi (Nagaokakyo, JP), Kondo;
Mitsushige (Nagaokakyo, JP) |
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
26466133 |
Appl.
No.: |
07/058,879 |
Filed: |
June 5, 1987 |
Foreign Application Priority Data
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|
|
|
Jun 6, 1986 [JP] |
|
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61-131236 |
Jun 20, 1986 [JP] |
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61-144038 |
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Current U.S.
Class: |
369/116; 369/121;
369/122; G9B/7.099 |
Current CPC
Class: |
G11B
7/126 (20130101) |
Current International
Class: |
G11B
7/125 (20060101); G11B 007/00 () |
Field of
Search: |
;369/124,121,122,116,43-47 ;358/342 ;346/762 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Richardson; Robert L.
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed is:
1. An optical type head device comprising:
a laser beam source for emitting a laser beam,
a light focussing means for converging said laser beam on a data
storing surface of an optical type data storing medium,
a laser beam detecting means which receives said laser beam
reflected by said optical type data storing medium and modulated by
data held by the same, and converts said data into electric
signals,
a driving current supplying means for supplying a driving current
to said laser beam source, wherein said driving current supplying
means comprises a high frequency current generator which supplys to
said laser beam source a high frequency current having a waveform
ranging from said lower part or said threshold value at which said
laser beam source starts emission of said laser beam to the higher
part of said threshold value, and the frequency of said high
frequency current is at least twice as high as said maximum
frequency component of information which is contained in said data
storing medium.
2. The optical type head device according to claim 1, wherein a
laser beam intensity detecting means is provided to detect the
intensity of said laser beam emitted from said laser beam source,
and said driving current supplying means produces an intensity
signal substantially in proportion to the peak intensity of said
emitted laser beam on the basis of the detected signal of said
laser beam intensity detecting means to control the amplitude of
said high frequency current supplied to said laser beam source on
the basis of said intensity signal, whereby the peak intensity of
said emitted laser beam is made constant.
3. The optical type head device according to claim 1, wherein a
laser beam intensity detecting means is provided to detect the
intensity of said laser beam emitted from said laser beam source,
and said driving current supplying means produces an average power
signal substantially in proportion to the average emission power of
said emitted laser beam on the basis of the detected signal of said
laser beam intensity detecting means to control the amplitude of
the high frequency current supplied to said laser beam source on
the basis of said average power signal, whereby the peak intensity
of said emitted laser beam is made constant.
4. The optical type head device according to claim 1, wherein said
high frequency current has a cyclic waveform varying between zero
and the peak value.
5. The optical type head device according to claim 1, wherein the
shape of the waveform of the high frequency current is
substantially rectangular.
6. The optical type head device according to claim 1, wherein a
photoeletric converting means is connected to said laser beam
detecting means to output a data reproducing signal on said data
storing medium by converting an output current from said laser beam
detecting means into a voltage, and the upper limit of an operable
frequency band for said photoelectric converting means is so
determined that a high frequency component of said driving current
to said laser beam source is not mixed with said reproducing
signal.
7. The optical type head device according to claim 1, which further
comprises a demodulating means to detect data in said electric
signal detected by said laser beam detecting means in synchronism
with a current signal from a high frequency current generator.
8. The optical type head device according to claim 7, wherein said
demodulating means has a sample-holding circuit operating in
synchronism with the current signal of said high frequency current
generator.
9. The optical type head device according to claim 7, wherein said
demodulating means has a filter which cuts a frequency component
higher than the maximum frequency in the data to be reproduced.
10. The optical type head device according to claim 7, wherein
there is provided a negative feedback control loop which averages
for detection the intensity of the signal of the laser beam emitted
from said laser beam source so that power to be supplied to said
laser beam source is stabilized by said average intensity
signal.
11. The optical type head device according to claim 7, wherein a
laser beam intensity detecting means is provided to detect the
intensity of said laser beam emitted from said laser beam source
subjected to a high frequency driving, whereby the amplitude of a
high frequency signal to be supplied to said laser beam source is
controlled to thereby make the peak intensity of said emitted laser
beam constant.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical type head device. More
particularly, it relates to an improvement in a driving current
supplying means for feeding an electric current to a semiconductor
laser beam source in the optical type head device.
2. Discussion of Background
There is well known an optical type head device for reading data
stored on an optical type data storing medium such as an optical
disc on which surface pits are formed, by supplying a laser beam
and by detecting the laser beam reflected by the optical disc. Such
optical type head device is used for various electric devices and
appliances such as an audio PCM disc player, a video disc player
and so on.
FIG. 14 shows general construction of a conventional optical type
head device. The device comprises a semiconductor laser beam source
1 (hereinbelow referred to as a light source) emitting a laser
beam, a beam splitter 2 for separating the emitted laser beam from
the reflected laser beam, the beam splitter 2 being formed by a
prism having a half-mirror portion, and a condensor lens 3 which
constitutes a light converging means.
In the rear of the laser beam source 1, provided is a light
detector 4 as a laser beam intensity detecting and
photoelectrically converting means which outputs an electric
current in proportion to the intensity of the laser beam emitted
from the laser beam source 1 and which is connected to a driving
power source 5 as a driving current supplying means so that an
electric current is supplied to the laser beam source 1. Also, an
optical disc 6 is provided at a position where the laser beam i
converged by means of the condensor lens 3. A motor-driven shaft 7
is fitted in an opening at the center of the optical disc 6 so that
it is rotated by a motor.
Further, a leser beam detector 9 is provided at a position where
the laser beam reflected by the data storing surface 8 of the
optical disc 6 is converged.
The laser beam detector 9 outputs an electric signal to a
pre-amplifier 10 in which the electric current signal is converted
into an electric voltage which is processed in a circuit (not
shown) to be utilized for an electric device such as an audio or a
video device or a digital player and so on.
The operation of the optical type head device having the
above-mentioned construction will be described.
A driving current is supplied to the leaser beam source 1 by the
driving power source 5 and the laser beam source 1 emitts a laser
beam to the beam splitter 2. At the same time, light 12 in
proportion to the intensity of the emitted laser beam 11 is
generated backwardly from the laser beam source 1. The light 12 is
detected by the monitoring light detector 4 which in turn,
generates an electric current in proportion to the intensity of the
emitted laser beam 11 to the driving power source 5 as a feedback
signal. Namely, the laser beam source 1, the light detector 4 and
the driving power source 5 constitute a negative feedback control
loop so that a constant laser beam output can be obtained even when
there is change in characteristics of the emitted laser beam to an
input current.
The laser beam 11 is passed through the beam splitter 2 and is
focussed on the data storing surface 8 of the optical disc 6 by
means of the condensor lens. Since the data storing surface 8 has a
number of pits, the laser beam focussed on the optical disc is
modulated by recorded data and is reflected to the condensor lens
3. The reflected laser beam passes through the lens 3 and changes
its path to the laser beam detector 9 by means of the beam splitter
2.
The laser beam detector 9 outputs an electric current depending on
the intensity of the reflected laser beam 13. The electric current
signal is converted into a voltage signal by a pre-amplifier 10 to
be outputted from a terminal 14.
As the optical disc 6 is rotated by the shaft 7, the data on the
optical disc 6 are succesively reproduced and outputted from the
terminal.
The optical type head device has usually a focussing servo means
for correcting focussing error of the laser beam on the optical
disc and a tracking servo means for correcting deviation in the
light spot of the laser beam to a data track on the optical disc
although these means are omitted in the drawings.
FIG. 5 illustrates operational characteristics of the laser beam
source in a relation of driving current I to the power of the
emitted laser beam.
As apparent from FIG. 15, the I-P characteristics of the laser beam
source 1 is in a non-linear form in which inclination of the
characteristic line suddenly increses when the driving current I
exceeds the threshold value Ith. Namely, the region of I>Ith is
a "laser oscillation region" and the region of 0<I<Ith is a
"spontaneous emission region" in which there is no oscillation of a
laser beam.
In the above-mentioned optical type head device, a coherent laser
beam is converged on the optical disc to read the data on the
optical disc. In this case, the driving current of the laser beam
source which contributes reading of the data is only in the region
of I>Ith which causes laser beam oscillation.
On the other hand, an operating power P.sub.DC is typically 3 mW
(Watts) in the case of the optical type head device having the I-P
characteristics shown in FIG. 15. In this case, the driving current
I.sub.DC is 40 mA (Ampares) and the threshold current Ith is 30 mA.
Accordingly, when the laser beam source is to be driven by
supplying 40 mA of the driving current I.sub.DC, only an electric
current of I.sub.DC -Ith=10 A is utilized as the current to produce
the laser beam necessary to operate the optical type head
device.
When I>Ith, change in the voltage across the PN junction
(forward operating voltage) of the laser beam source is extremely
small in the maximum rated current range, which can be regarded as
being practically constant. Specifically, a voltage value is 1.8 V
(Volts) in a device having the characteristics shown in FIG.
15.
In the conventional optical type head device, a d.c. current having
a constant value is used to drive the laser beam source. As
described before, since the I-P characteristics is in a non-linear
form, a proportion of the threshold current Ith to the driving
current I.sub.DC is large, whereby a large driving power is
required to operate the laser beam source. The large driving power
increases temperature at the PN junction of the laser beam source
to thereby shorten the lifetime of it. There is another problem
that the lifetime of a battery is shortened when it is installed in
an electric device such as a portable type CD player.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical type
head device which reduces a driving power from a laser beam source
without deterioration of the reproduction characteristics and
extends the lifetime of the laser beam source while saving
consumption power.
The foregoing and the other objects of the present invention have
been attained by providing an optical type head device comprising a
laser beam source for emitting a laser beam, a light focussing
means for converging the laser beam on a data storing surface of an
optical type data storing medium, a laser beam detecting means
which receives the laser beam reflected by the optical type data
storing medium and modulated by data held by the same, and converts
the data into electric signals, a driving current supplying means
for supplying a driving current to the laser beam source, wherein
the driving current supplying means comprises a high frequency
current generator which supplys to the laser beam source a high
frequency current having a waveform ranging from the lower part of
the thresheld value at which the laser beam source starts emission
of the laser beam to the higher part of the thresheld value, and
the frequency of the high frequency current is at least twice as
high as the maximum frequency component of information which is
contained in the data storing medium.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the
attendant advantages thereof will be readily obtained as the same
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings, wherein:
FIG. 1 is a diagram showing a first embodiment of the optical type
head device according to the present invention;
FIGS. 2 (A) through (C) is a diagram showing the function of the
head device of the first embodiment;
FIG. 3 is a diagram showing reduction of consumption power in the
head device of the first embodiment;
FIGS. 4 (A) and (B) diagram are showing operation of reading data
on an optical disc;
FIG. 5 is a diagram showing a second embodiment of the optical type
head device according to the present invention;
FIG. 6 is a diagram showing a driving current supplying means used
for a third embodiment of the optical type head device according to
the present invention;
FIG. 7 is a diagram showing a fourth embodiment of the optical type
head device according to the present invention;
FIG. 8 is a diagram showing operational characteristics of the
optical type head device of the present invention;
FIGS. 9a to 9c show waveforms in the step of demodulation of
electric signals to be reproduced;
FIG. 10 is a diagram showing reduction of a driving power in the
present invention;
FIG. 11 is a diagram showing another system of driving the laser
beam source of the present invention;
FIG. 12 is a diagram showing a fifth embodiment of the optical type
head device according to the present invention;
FIG. 13 is a diagram showing a modification of an important part in
the optical type head device shown in FIG. 12;
FIG. 14 is a diagram showing a conventional optical type head
device; and
FIG. 15 is a diagram showing operational characteristics of the
conventional device.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Several preferred embodiments of the optical type head device
according to the present invention will be described with reference
to the drawings.
FIG. 1 shows a general construction of the optical type head device
according to the first embodiment of the designate the same or
corresponding parts, and therefore, description of these parts is
omitted.
A driving current supplying means 21 electrically connected to the
laser beam source 1 comprises a high frequency current oscillator
22 which produces a high frequency current whose amplitude spans a
current region which covers the threshold value at which the laser
beam source starts laser oscillation and a current region which is
lower than the threshold value. A low-pass filter 23 is connected
to the output side of the pre-amplifier 10.
The operation of the optical type head device of the first
embodiment will be described.
The high frequency current generator 22 of the current supplying
means 21 outputs a high frequency current having a frequency fp to
the laser beam source 1. The high frequency current is a series of
pulses in a rectangular form having a duty ratio of 0.5 which raise
and fall between zero and I.sub.P (I.sub.P >Ith).
FIG. 2 illustrates the function of the laser beam source 1.
FIG. 2A shows I-P characteristics of the laser beam source 1; FIG.
2B shows the waveform of the driving current for the laser beam
source 1, and FIG. 2C shows the waveform of a power P for emitting
the laser beam.
In more detail, when the driving current shown in FIG. 2B is
supplied to the laser beam source 1, the laser beam is emitted from
the laser beam source 1 in which the laser beam is in the
rectangular waveform having a power P, a frequency fp and a duty
ratio of 0.5, and in which the laser beam emission power changes
from zero to P.sub.P. In FIG. 2, I.sub.DC and P.sub.DC respectively
refer to a d.c. driving current having a constant value and the
power of the laser beam in the conventional optical type head
device. When the duty ratio of the pulse current is 0.5, the
average value of the power of the laser beam is one half of the
peak power P.sub.p. Accordingly, by selecting a peak current
I.sub.P to satisfy a relation of P.sub.P =2P.sub.DC, a relation of
P.sub.P =P.sub.DC is obtainable, whereby the average power of the
pulsed laser beam P.sub.P is in coincidence with the driving power
P.sub.DC as if the constant current is supplied.
In the following, consideration is made as to an average driving
current I.sub.P in a state that the average power of the light beam
P.sub.P is made in agreement with the driving power P.sub.DC by the
constant current.
As shown in FIG. 2, when the duty ratio is 0.5, the average driving
current I.sub.P is one-half of I.sub.p. In consideration of the
non-linear I-P characteristics, when I.sub.p >Ith, a relation
I.sub.P <I.sub.DC is always established. Thus, the phenomenon
that the average current I.sub.P is smaller than I.sub.DC may be
obtained as long as the laser beam source is not broken due to the
peak current I.sub.P ever though the duty ratio is not 0.5.
The phenomenon as described above can be understood as follows.
In an ON period during which the driving current I.sub.P in a pulse
form is supplied to the laser beam source 1, the rate of I.sub.P to
Ith is remarkably lower than the rate of I.sub.DC to Ith.
Accordingly, there is formed increase of a proportion of the
current usuable for laser oscillation for the total amount of the
current to be supplied to the laser beam source. Namely, in the
conventional device, a current component of Ith with respect to the
driving current I.sub.DC does not contribute to the laser
oscillation. However, in the present invention, a current component
of a factor of (1-DR) can be reduced (where DR referred to a value
of the duty in the waveform of the driving current and it is
expressed by b/a in FIG. 2B).
In FIG. 2, driving current dependency of a forward voltage of the
laser beam source is extremely small in the region of I>Ith and
it can be considered to be substantially constant. Accordingly, a
ratio of an average power for the laser beam source 1 is I.sub.DC
:I.sub.P when the laser beam source 1 is driven by the pulse
current.
FIG. 3 is a diagram which shows that an average power can be
reduced in the I-P characteristics shown in FIG. 15. In FIG. 15,
laser beam emission power P.sub.DC is 3 mW, and the I-P
characteristics are approximately defined by two linear lines
connected at Ith.
In FIG. 3, when the laser beam source is actuated by the pulse
current, a power of 6 mW is permissible for the peak power P.sub.P.
A range of the duty ratio to provide the peak power P.sub.P is
0.5-1.0, the area being hatched in FIG. 3. Accordingly, it is
understandable that 40% reduction in the driving power can be
attained by utilizing the pulse duty ratio at the point A in the
above-mentioned range.
The peak power allowable for the laser beam source is variable
depending on the nature of laser beam sources to be used. Also, the
range of 0.5-1.0 of the duty ratio which has been mentioned to be
desirable is for an example, and if there is a laser beam which
permits a greater peak power P.sub.P, a smaller duty ratio can be
selected.
The operation for reading the data of the optical type head device
in which the pulse-current-driven laser beam source 1 is used will
be described.
The laser beam 11 emitted from the laser beam source 1 is passed
through the beam splitter 2 and is converged on the data storing
surface 8 of the optical disc 6 through the condensor lens 3. The
laser beam is reflected by the data storing surface 8 and is
retuned to the beam splitter 2 through the lens 3. The laser beam
is reflected by the beam splitter 2 to be received in the laser
beam detector 9. The laser beam is modulated by the data recorded
on the optical disc 6 which is rotated by the rotating shaft 7, and
the modulated laser beam is subjected to photoelectric conversion
by the detector 9, and thus detected output is entered in the
pre-amplifier. As shown in FIG. 4A, the output of the pre-amplifier
10 is a pulse form which is sampled with a time interval which is
determined by a repetition frequency fp of the pulse, and a chain
line obtained by connecting each peak of the pulse constitutes
information of the data to be read.
The low-pass filter 23 has a cut-off frequency fc (fc<fp) to
take out data components as indicated by the chain line.
Accordingly, a reproduced output as shown in FIG. 4B is obtainable
from the output terminal 14. The amplitude of the output can be
considered to be the same as the reproduced output obtained by
feeding the constant electric current if the average laser beam
output is equal to the case of the constant electric current.
When the maximum frequency of the data recorded in the optical disc
is fi, the frequency fp of the laser beam in the pulse form must be
at least 2fi from the "sampling theory".
The inventors of the present application conducted experiments to
read out the data by feeding a pulse current by using a compact
disc as a optical disc.
The frequency fp of the pulse current was determined to be 19 MHz
and the output of the pre-amplifier 10 was observed. It was found
that a signal of 19 MHz was overlapped in a saw-toothed form on a
reproduced waveform (the maximum frequency was about 720 KHz) of a
compact disc.
A primary low-pass filter (fc=6.8 MHz) was inserted as the low-pass
filter 23. As a result, a reproduced waveform substantially equal
to that obtained when the constant current is supplied, was
observed.
The low-pass filter 23 are not necessarily separated from the
pre-amplifier 10. If the upper limit of the frequency band of the
pre-amplifier is low to sufficiently remove pulse components of the
frequency fp, it is unnecessary to insert the low-pass filter
23.
As described before, fp can be selected to have a relation of
fp.gtoreq.2fi by the sampling theory. However, when the band of the
pre-amplifier is to be limited by a low-pass filter having a phase
distortion, phase distortion of the reproduced signal takes place
at the vicinity of a cut-off frequency fc. In this case, an
excellent reproduction characteristic may not be obtained when fp
is determined to be nearly 2fi. Accordingly, it is desirable to
select fp to have relation of fp.gtoreq.3fi to obtain an excellent
reproduction characteristics when the band description is required
by using an easy way such as the primary low-pass filter consisting
of CR except that a perfect low-pass filter free from the phase
distortion is used.
In the following, a second embodiment of the optical type head
device of the present invention will be described with reference to
FIG. 5.
In contrast with the first embodiment in which the driving current
supplying means 21 having the high frequency current generator 22
is connected to the laser beam source 1 to supply the pulse
current, the second embodiment is so adapted that the peak power of
the laser beam source 1 is made constant by the current supplying
means 21. Namely, the current supplying means 21 of the second
embodiment comprises a high frequency current generator 22 having
an input terminal 22a for controlling the amplitude of the pulse
current, a light detector 4 for monitoring the intensity of the
laser beam emitted from the laser beam source 1, an I/V transducer
30 for converting an output current from the light detector 4 into
an electric voltage, and a sample-hold circuit for detecting a peak
output from the I/V transducer 30, i.e. the most intense point of
the output of the pulse-modulated laser beam from the laser beam,
by an input signal inputted in a sampling- instruction-signal input
terminal 32a so as to be in synchronism with the output of the high
frequency current generator 22. Accordingly, a power for actuating
the laser beam source is controlled by a control loop shown in FIG.
5, so that the peak value of the output of the laser beam is
stabilized.
According to the above-mentioned embodiments, the peak intensity of
the pulse laser beam can be kept constant even when there is change
in ambient temperature and the I-P characteristics of the laser
beam source. It is also possible to avoid a short lifetime of the
laser beam source due to increase in the peak value of the pulse
current or reduction of the amplitude of a reproduced signal due to
reduction of the peak value.
FIG. 6 shows a diagram of the driving current supplying circuit 21
of the optical type head device according to the third embodiment
of the present invention.
The third embodiment is so constructed that an output of the light
detector 4 is subjected to I/V conversion and thus obtained voltage
signal is inputted in a low-pass filter 40 so that the output of
the low-pass filter is supplied for negative feedback to an
amplitude controlling input terminal 22a. The low-pass filter 40
has a cut-off frequency which is sufficiently lower than the
frequency fp of the pulse current from the high frequency current
generator 22 so as to average the output from the I/V
transducer.
Since the average output power from the low-pass filter 40 has a
co-relation with the peak power of the laser beam source 1, the
peak power is kept constant by the above-mentioned control
loop.
The third embodiment has an advantage of more simplified
construction in comparison with the second embodiment in which the
peak power itself is sampled and is used as a feedback signal.
FIG. 7 is a diagram showing a fourth embodiment of the optical type
head device according to the present invention. In FIG. 7, the same
reference numerals as in FIG. 1 designate the same or corresponding
parts, and therefore, only parts different from those of the first
embodiment will be described.
A reference numeral 60 designates a driving unit in which a high
frequency pulse signal generator 50 is installed to supply a pulse
signal having a predetermined high frequency to the laser beam
source 1.
The laser beam detector 9 outputs an electric current signal having
the intensity depending on the laser beam 13 reflected by the data
storing surface 8. The output current is subjected to I/V
conversion in the pre-amplifier 52; and then, are passed through a
sample-hold circuit 53 and a low-pass filter 54, and is finally
outputted from an output terminal 58. The sample-hold circuit 53
and the low-pass filter 54 constitute a demodulating means 70.
Thus, an electrically reproduced signal is outputted as a form of
voltage function of the data stored in the data storing surface 8
of the optical disc 6.
In the fourth embodiment, the light detector 10 for monitoring the
intensity of the laser beam emitted from the laser beam source 1 is
not utilized. The fourth embodiment includes the focussing servo
and tracking servo systems which are similar to or the same as the
first embodiment. But, they are also omitted in the drawings. In
the optical type head device shown in FIG. 7, a laser beam source
actuating device 21 is provided with a high frequency pulse
oscillator 22 so that a driving current having a high frequency is
supplied to the laser beam source 1. The demodulating means 70 is
provided between the pre-amplifier 52 and the output terminal 58.
The output signal 55 of the pre-amplifier 52 is sampled by the
sample-hold circuit 53 so as to be in synchronism with the
frequency of the high frequency current 57 which is supplied from
the laser beam source actuating device 21 to the laser beam source
1. The output 56 of the sample-hold circuit 53 is smoothed by the
low-pass filter 54 to thereby remove frequency components of the
sampled signal, and the output of the low-pass filter 54 is
outputted from the output terminal 58. Thus, the readable data
containing in the optical signal received by the laser beam
detector 9 are detected in synchronism with the frequency of the
high frequency current 57 from the laser beam source actuating
device 21.
The operation of the fourth embodiment of the optical type head
device will be described.
FIG. 8 shows at its lower portion a waveform of the driving circuit
I outputted from the laser beam source actuating device 21 shown in
FIG. 7. The laser beam source actuating device 21 outputs a high
frequency current having a rectangular waveform with a repetition
frequency of fp and a duty of D.sub.R (D.sub.R =b/a) which spans
the regions between zero currnet (I=0) and a peak current Ip whch
is grater than the threshold current Ith. The driving current I is
supplied to the laser beam source.
The waveform illustrated in the right part of FIG. 8 is a waveform
of the laser beam power P from the laser beam source 1 when it is
actuated by the driving current I. The laser beam power is a pulse
form having a rectangular waveform having the same repetition
frequency fp and duty D.sub.R as those of the driving current I.
Namely, the laser beam power p of the laser beam source is
modulated by the high frequency power of the driving current I so
as to cause oscillation between the zero power (P=0) and the peak
power (P=P.sub.P) In this case, the magnitude of the peak current
I.sub.P is determined to have the same magnitude as the constant
current I.sub.DC obtained when the laser beam source is actuated by
a d.c. current. Thus, the laser beam power P.sub.DC having the same
level as that when the laser beam source is actuated by the
constant d.c. current I.sub.DC is outputted.
FIGS. 9a, 9b and 9c respectively show the waveform of the outputs
from the pre-amplifier 52, the sample-hold circuit 53 and the
low-pass filter 54. In the Figures, GND refer to ground
potential.
Since the laser beam power p of the laser beam source 1 undergoes
the primary modulation by the high frequency pulse, the output 55
of the pre-amplifier 52 is in a pulse form having a repetition
frequency of fp (fp=1/TP) The laser beam of the pulse form is
subjected to the secondary modulation by the data stored on the
optical disc, whereby the data taken out from the optical disc is
in an envelopeline formed by connecting each peak of the pulsated
waveform as shown in FIG. 9a. Namely, the envelopeline of the
output 55 of the pre-amplifier 52 shows reproduced data from the
optical disc. The envelopeline is sampled by the sample-hold
circuit 53 in synchronism with the high frequency pulse, whereby
the envelopeline including the recorded data of the optical disc is
obtained from the sample-hold circuit 53 by synchronous detection.
The output 56 of the sample-hold circuit 53 is a stepwisely
changing waveform as shown in FIG. 9b in which high frequency
components other than a frequency component of the data to be
reproduced. The needless high frequency components are removed by
passing the electric signal through the low-pass filter 54. In this
case, the cut-off frequency fc of the low-pass filter 54 is
determined to be higher than a frequency region of the data to be
reproduced and lower than the frequency fp of the driving current
I, whereby reproduction signals having amplitudes which change
depending on those of the recorded data on the optical disc are
outputted from the low-pass filter 54 to the output terminal
58.
The frequency fp of the driving current I may be determined to be
two times as the maximum frequency fi of the reproduced data
according to the well-known sampling theory. However, if frequency
cutting-off characteristics of the low-pass filter 54 in the
vicinity of the cut-off frequency fc is not sharp, there may cause
distortion in the reproduced signals. Accordingly, it is desirable
that some allowance should be given to the frequency fp of the
driving current when a simple low-pass filter such as a combination
of a resistor amd a capasitor is used. It is desirable from a
practical viewpoint that the frequency to be used is in a range of
fp.gtoreq.3fi.
By providing the construction as mentioned above, the laser beam
source is intermittently actuated at a relatively short repeating
period (1/fp). Since supply of a driving power to the laser beam
source is intermittently stopped, an average driving power required
throughout the entire operating time can be remarkably reduced in
comparison with the case driven by a direct current, even though
the peak value of the power is made large.
On the other hand, reading of the data by the laser beam from the
laser beam source is carried out only in the range in which the
intensity of the laser beam becomes periodically maximum.
Accordingly, the data on the optical disc can be certainely read
out in the same manner as in the case that a sufficiently large
d.c. driving current is constantly supplied to the laser beam
source.
FIG. 10 is a diagram showing a relation between variation in a duty
D.sub.R of a high frequency pulse current and effect of reducing a
driving power for a laser beam source. In this cae, a reduction
rate of the driving power is expressed by a ratio of an average
consumption power obtained when the laser beam source is actuated
by a pulse current from a high frequency power source to an average
consumption power obtained when the laser beam source is actuated
by a d.c. current from a d.c. power source (a power actuated by a
pulse current / a power actuated by d.c. current).
As clear from FIG. 10, when a duty ratio D.sub.R of the pulse
current is 0.5, for instance, the average power is only about half
in comparison with the case that a d.c. current is continuously
supplied. The smaller the duty ratio D.sub.R is, the grater the
effect of reducing the driving power is. The duty ratio can be
small within a limit of, for instance, the minimal sampling time
(settling time) of a sample-hold circuit.
With reference to FIG. 11, the driving current I for the laser beam
source is not always necessary to change the driving current I in a
pulse form values over the entire region from the current zero
level to the peak value Ip. The driving current I may be in the
range between the peak current value I.sub.P and a current value
lower than the threshold value Ith, whereby a power of the laser
beam P having an amplitude between substantially zero level portion
and the peak level P.sub.P can be obtained. Thus, by controlling
the amplitude of the driving current I, a load for the high
frequency pulse current generator can be reduced.
FIG. 12 is a diagram showing a fifth embodiment of the optical type
head device according to the present invention.
The characteristic feature of this embodiment is to provide a
current/voltage transducer 82 and a sample-hold circuit 83 in a
laser beam source actuating device 90. The current/voltage
transducer 82 is connected to the light detector 4 for monitoring
the intensity of light 12 which is backwardly emitted from the
laser beam source 1, whereby an electric signal having the
intensity which changes depending on the laser beam 11 emitted
forwardly from the laser beam source 1. The output signal from the
transducer 82 is inputted into the sample-hold circuit 83 which
performs a sampling operation in synchronism with the frequency of
a high frequency pulse current 84 which is given from a high
frequency pulse generator 22 to the laser beam source 1. The
sampled output signal 85 is fedback to the pulse generator 22.
Namely, a negative feedback control loop is constituted by the
light detector 4, the current/voltage transducer 82, the
sample-hold circuit 83, the pulse generator 22 and the laser beam
source 1 to stablize the peak power of the laser beam 11 of the
laser beam source 1. Accordingly, a constant peak power of the
laser beam is always obtainable even when there is change in
temperature and characteristics of the laser beam source 1. For
instance, reduction of the lifetime of the laser beam source due to
undesired rise of the peak power of the laser beam, or reduction of
the reproducing function due to the reduction in the peak power of
the laser beam can be avoided.
FIG. 13 is a diagram showing a separate embodiment of the optical
type head device in which a negative feedback control loop for
stablizing the laser beam 11 of the laser beam source 1 is
illustrated.
The negative feedback control loop is so constructed that an output
signal for monitoring the intensity of the laser beam produced from
the current/voltage transducer 102 is averaged by a low-pass filter
110, and the averaged signal is fedback to a pulse generator 22. In
this case, the low-pass filter 110 is so constructed to have a
cut-off frequency sufficiently lower than the frequency of a high
frequency pulse current for actuating the laser beam source 1,
whereby the peak power of the laser beam can be simply stabilized
without using a sample-hold circuit.
In the above-mentioned embodiments, a driving current having a sine
waveform, a trianglar waveform or a similar waveform may be used
instead of the driving current having a rectangular waveform as far
as the driving current covers the threshold current Ith region in
the I-P characterstics of a laser beam source.
An output having a bi-polarity oscillation voltage may be used
instead of a single polarity oscillation voltage. In this case, a
current flows only in the forward direction by switching function
of the laser beam source. The function is the same.
The present invention is applicable to another useage other than
actuation of the laser beam source which is used in an optical head
device as mentioned above, for instance, it is applicable to an
optical type sensor; which contains laser diode as a light source,
so that the driving power of the laser beam source can be
reduced.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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